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6: Sex Influences the Inheritance and Expression of Genes in a Variety of Ways
Extensions and Modifications of Basic Principles
homozygous recessive males will have the cock-feathering
Sex-influenced characteristics are encoded by autosomal genes
that are more readily expressed in one sex. Sex-limited characteristics are encoded by autosomal genes whose expression is limited
to one sex.
✔ Concept Check 11
How do sex-influenced and sex-limited traits differ from sex-linked
Mendel’s principles of segregation and independent assortment are based on the assumption that genes are located on
chromosomes in the nucleus of the cell. For most genetic
characteristics, this assumption is valid, and Mendel’s principles allow us to predict the types of offspring that will be
produced in a genetic cross. However, not all the genetic
material of a cell is found in the nucleus; some characteristics are encoded by genes located in the cytoplasm. These
characteristics exhibit cytoplasmic inheritance.
A few organelles, notably chloroplasts and mitochondria, contain DNA. Each human mitochondrion contains
about 15,000 nucleotides of DNA, encoding 37 genes.
Compared with that of nuclear DNA, which contains some
3 billion nucleotides encoding perhaps 25,000 genes, the
amount of mitochondrial DNA (mtDNA) is very small; nevertheless, mitochondrial and chloroplast genes encode some
Cytoplasmic inheritance differs from the inheritance of
characteristics encoded by nuclear genes in several important respects. A zygote inherits nuclear genes from both parents; but, typically, all its cytoplasmic organelles, and thus all
its cytoplasmic genes, come from only one of the gametes,
usually the egg. A sperm generally contributes only a set of
nuclear genes from the male parent. In a few organisms,
cytoplasmic genes are inherited from the male parent or
from both parents; however, for most organisms, all the cytoplasm is inherited from the egg. In this case, cytoplasmically
inherited traits are present in both males and females and are
passed from mother to offspring, never from father to offspring. Reciprocal crosses, therefore, give different results
when cytoplasmic genes encode a trait.
Cytoplasmically inherited characteristics frequently
exhibit extensive phenotypic variation, because no mechanism analogous to mitosis or meiosis ensures that cytoplasmic genes are evenly distributed in cell division. Thus,
different cells and individual offspring will contain various
proportions of cytoplasmic genes.
Consider mitochondrial genes. Most cells contain thousands of mitochondria, and each mitochondrion contains
This cell contains an
equal number of
wild-type genes and
in cell division…
in cell division.
Replication of mitochondria
Replication of mitochondria
...results in progeny cells that differ in their number of
mitochondria with wild-type and mutated genes.
4.19 Cytoplasmically inherited characteristics frequently
exhibit extensive phenotypical variation because cells
and individual offspring contain various proportions of
cytoplasmic genes. Mitochondria that have wild-type mitochondrial
DNA are shown in red; those having mutant mtDNA are shown in blue.
from 2 to 10 copies of mtDNA. Suppose that half of the
mitochondria in a cell contain a normal wild-type copy of
mtDNA and the other half contain a mutated copy (Figure
4.19). In cell division, the mitochondria segregate into progeny cells at random. Just by chance, one cell may receive
mostly mutated mtDNA and another cell may receive mostly
wild-type mtDNA. In this way, different progeny from the
same mother and even cells within an individual offspring
may vary in their phenotypes. Traits encoded by chloroplast
DNA (cpDNA) are similarly variable.
In 1909, cytoplasmic inheritance was recognized by Carl
Correns as one of the first exceptions to Mendel’s principles.
Correns, one of the biologists who rediscovered Mendel’s
work, studied the inheritance of leaf variegation in the fouro’clock plant, Mirabilis jalapa. Correns found that the leaves
and shoots of one variety of four-o’clock were variegated,
displaying a mixture of green and white splotches. He also
noted that some branches of the variegated strain had allgreen leaves; other branches had all-white leaves. Each
Question: How is stem and leaf color inherited in the
Pollen plant (
Cross flowers from
white, green, and
in all combinations.
Seed plant (
Correns’s crosses demonstrated cytoplasmic inheritance
of variegation in the four-o’clocks. The phenotypes of the
offspring were determined entirely by the maternal parent,
never by the paternal parent (the source of the pollen).
Furthermore, the production of all three phenotypes by
flowers on variegated branches is consistent with cytoplasmic inheritance. Variegation in these plants is caused by a
defective gene in the cpDNA, which results in a failure to
produce the green pigment chlorophyll. Cells from green
branches contain normal chloroplasts only, cells from white
branches contain abnormal chloroplasts only, and cells from
variegated branches contain a mixture of normal and abnormal chloroplasts. In the flowers from variegated branches,
the random segregation of chloroplasts in the course of
oogenesis produces some egg cells with normal cpDNA,
which develop into green progeny; other egg cells with only
abnormal cpDNA develop into white progeny; and, finally,
still other egg cells with a mixture of normal and abnormal
cpDNA develop into variegated progeny.
A number of human diseases (mostly rare) that exhibit
cytoplasmic inheritance have been identified. These disorders arise from mutations in mtDNA, most of which occur
in genes encoding components of the electron-transport
chain, which generates most of the ATP (adenosine triphosphate) in aerobic cellular respiration. One such disease is
Leber hereditary optic neuropathy (LHON). Patients who
have this disorder experience rapid loss of vision in both
eyes, resulting from the death of cells in the optic nerve. This
loss of vision typically occurs in early adulthood (usually
between the ages of 20 and 24), but it can occur any time
after adolescence. There is much clinical variability in the
severity of the disease, even within the same family. Leber
hereditary optic neuropathy exhibits maternal inheritance:
the trait is always passed from mother to child.
Genetic Maternal Effect
Conclusion: The phenotype of the progeny is determined by
the phenotype of the branch from which the seed originated,
not from the branch on which the pollen originated. Stem and
leaf color exhibits cytoplasmic inheritance.
4.20 Crosses for leaf type in four-o’clocks illustrate
branch produced flowers; so Correns was able to cross flowers from variegated, green, and white branches in all combinations (Figure 4.20). The seeds from green branches always
gave rise to green progeny, no matter whether the pollen was
from a green, white, or variegated branch. Similarly, flowers
on white branches always produced white progeny. Flowers
on the variegated branches gave rise to green, white, and variegated progeny, in no particular ratio.
A genetic phenomenon that is sometimes confused with
cytoplasmic inheritance is genetic maternal effect, in which
the phenotype of the offspring is determined by the genotype of the mother. In cytoplasmic inheritance, the genes for
a characteristic are inherited from only one parent, usually
the mother. In genetic maternal effect, the genes are inherited from both parents, but the offspring’s phenotype is
determined not by its own genotype but by the genotype of
Genetic maternal effect frequently arises when substances present in the cytoplasm of an egg (encoded by the
mother’s nuclear genes) are pivotal in early development. An
excellent example is the shell coiling of the snail Limnaea
peregra (Figure 4.21). In most snails of this species, the shell
coils to the right, which is termed dextral coiling. However,
some snails possess a left-coiling shell, exhibiting sinistral
coiling. The direction of coiling is determined by a pair of
alleles; the allele for dextral (sϩ) is dominant over the allele
for sinistral (s). However, the direction of coiling is deter-
Extensions and Modifications of Basic Principles
1 Dextral, a right-handed coil, results from
an autosomal allele (s+) that is dominant…
2 …over an allele
for sinistral (s),
a left-handed coil.
3 All the F1 are heterozygous
(s+s); because the genotype
of the mother determines the
phenotype of the offspring,
all the F1 have a sinistral shell.
Characteristics exhibiting cytoplasmic inheritance are encoded by
genes in the cytoplasm and are usually inherited from one parent,
most commonly the mother. In genetic maternal effect, the genotype of the mother determines the phenotype of the offspring.
dextral coiled because the genotype of their mother (sϩs)
encodes a right-coiling shell and determines their phenotype. With genetic maternal effect, the phenotype of the
progeny is not necessarily the same as the phenotype of the
mother, because the progeny’s phenotype is determined by
the mother’s genotype, not her phenotype. Neither the male
parent’s nor the offspring’s own genotype has any role in the
offspring’s phenotype. However, a male does influence the
phenotype of the F2 generation: by contributing to the genotypes of his daughters, he affects the phenotypes of their offspring. Genes that exhibit genetic maternal effect are
therefore transmitted through males to future generations.
In contrast, genes that exhibit cytoplasmic inheritance are
always transmitted through only one of the sexes (usually the
Conclusion: Because the mother of the F2 progeny
has genotype s+s, all the F2 snails are dextral.
4.21 In genetic maternal effect, the genotype of the
maternal parent determines the phenotype of the
offspring. The shell coiling of a snail is a trait that exhibits
genetic maternal effect.
mined not by that snail’s own genotype, but by the genotype
of its mother. The direction of coiling is affected by the way
in which the cytoplasm divides soon after fertilization, which
in turn is determined by a substance produced by the mother
and passed to the offspring in the cytoplasm of the egg.
If a male homozygous for dextral alleles (sϩsϩ) is
crossed with a female homozygous for sinistral alleles (ss), all
of the F1 are heterozygous (sϩs) and have a sinistral shell,
because the genotype of the mother (ss) encodes sinistral
coiling (Figure 4.21). If these F1 snails are self-fertilized, the
genotypic ratio of the F2 is 1 sϩsϩ : 2 sϩs : 1 ss.
Notice that that the phenotype of all the F2 snails is dextral coiled, regardless of their genotypes. The F2 offspring are
A basic tenet of Mendelian genetics is that the parental origin of a gene does not affect its expression and, therefore,
reciprocal crosses give identical results. However, the
expression of some genes is significantly affected by their
parental origin. This phenomenon, the differential expression of genetic material depending on whether it is inherited from the male or female parent, is called genomic
A gene that exhibits genomic imprinting in both mice
and humans is Igf 2, which encodes a protein called insulinlike growth factor II (Igf-II). Offspring inherit one Igf 2
allele from their mother and one from their father. The
paternal copy of Igf 2 is actively expressed in the fetus and
placenta, but the maternal copy is completely silent (Figure
4.22). Both male and female offspring possess Igf 2 genes;
the key to whether the gene is expressed is the sex of the
parent transmitting the gene. In the present example, the
gene is expressed only when it is transmitted by a male
parent. In other genomically imprinted traits, the trait is
expressed only when the gene is transmitted by the female
Genomic imprinting is brought about through differential methylation of DNA—the addition of methyl (CH3)
groups to DNA nucleotides. In mammals, methylation is
erased in the germ cells each generation and then reestablished
during gamete formation, with different levels of methylation
occuring in sperm and eggs, which then causes the differential
expression of male and female alleles in the offspring.
The paternal allele is
active and its protein
The maternal allele is
silent. The absence of
its protein product
does not further
stimulate fetal growth.
The size of the fetus
is determined by the
combined effects of
Genomic imprinting is just one form of a phenomenon
known as epigenetics, in which reversible changes to DNA
influence the expression of traits. Some of the ways in which
sex interacts with heredity are summarized in Table 4.5.
In genomic imprinting, the expression of a gene is influenced by
the sex of the parent that transmits the gene to the offspring.
Epigenetic marks are reversible changes to DNA that do not alter
the base sequence but may affect how a gene is expressed.
Sex influences on heredity
Phenotype determined by
Genes located on the sex
Sex-influenced characteristic Genes on autosomal chromosomes that are more readily
expressed in one sex
Autosomal genes whose
expression is limited to
Genetic maternal effect
Nuclear genotype of the
Cytoplasmic genes, which are
usually inherited entirely from
only one parent
Genes whose expression is
affected by the sex of the
4.22 Genomic imprinting of the lgf2 gene in mice and
humans affects fetal growth. (a) The paternal lgf2 allele is active
in the fetus and placenta, whereas the maternal allele is silent. (b) The
human lgf2 locus is on the short arm of chromosome 11; the locus in
mice is on chromosome 7. [Courtesy of Dr. Thomas Ried and Dr. Evelin
4.7 The Expression of a
Genotype May Be Influenced
by Environmental Effects
In Chapter 3, we learned that each phenotype is the result of a
genotype developing within a specific environment; the genotype sets the potential for development, but how the phenotype actually develops within the limits imposed by the
genotype depends on environmental effects. Stated another
way, each genotype may produce several different phenotypes,
depending on the environmental conditions in which development takes place. For example, a fruit fly homozygous for
the vestigial mutation (vg vg) develops reduced wings when
raised at a temperature below 29°C, but the same genotype
develops much longer wings when raised at 31°C. The range
of phenotypes (in this case, wing length) produced by a genotype in different environments is called the norm of reaction.
For most of the characteristics discussed so far, the effect
of the environment on the phenotype has been slight.
Mendel’s peas with genotype yy, for example, developed green
endosperm regardless of the environment in which they were
raised. Similarly, persons with genotype IAIA have the A antigen on their red blood cells regardless of their diet, socioeconomic status, or family environment. For other phenotypes,
however, environmental effects play a more important role.
on Gene Expression
The expression of some genotypes critically depends on
the presence of a specific environment. For example, the
himalayan allele in rabbits produces dark fur at the
extremities of the body—on the nose, ears, and feet
Extensions and Modifications of Basic Principles
4.23 The expression of some genotypes
Reared at 20°C or less
Reared at temperatures above 30°C
(Figure 4.23). The dark pigment develops, however, only
when the rabbit is reared at a temperature of 25°C or less;
if a Himalayan rabbit is reared at 30°C, no dark patches
develop. The expression of the himalayan allele is thus
temperature dependent; an enzyme necessary for the production of dark pigment is inactivated at higher temperatures. The pigment is restricted to the nose, feet, and ears
of a Himalayan rabbit because the animal’s core body temperature is normally above 25°C and the enzyme is functional only in the cells of the relatively cool extremities.
The himalayan allele is an example of a temperaturesensitive allele, an allele whose product is functional only
at certain temperatures.
Environmental factors also play an important role in
the expression of a number of human genetic diseases.
Glucose-6-phosphate dehydrogenase is an enzyme that
helps to supply energy to the cell. In humans, there are a
number of genetic variants of glucose-6-phosphate dehydrogenase, some of which destroy red blood cells when the
body is stressed by infection or by the ingestion of certain
drugs or foods. The symptoms of the genetic disease,
called glucose-6-phosphate dehydrogenase deficiency,
appear only in the presence of these specific environmental factors.
These examples illustrate the point that genes and their
products do not act in isolation; rather, they frequently interact with environmental factors. Occasionally, environmental
factors alone can produce a phenotype that is the same as the
phenotype produced by a genotype; this phenotype is called
a phenocopy. In fruit flies, for example, the autosomal recessive mutation eyeless produces greatly reduced eyes. The eyeless phenotype can also be produced by exposing the larvae
of normal flies to sodium metaborate.
The expression of many genes is modified by the environment. The
range of phenotypes produced by a genotype in different environments is called the norm of reaction. A phenocopy is a trait produced by environmental effects that mimics the phenotype
produced by a genotype.
depends on specific environments. The
expression of a temperature-sensitive allele,
himalayan, is shown in rabbits reared at different
The Inheritance of Continuous
So far, we’ve dealt primarily with characteristics that have
only a few distinct phenotypes. In Mendel’s peas, for example, the seeds were either smooth or wrinkled, yellow or
green; the coats of dogs were black, brown, or yellow; blood
types were of four distinct types, A, B, AB, or O. Such characteristics, which have a few easily distinguished phenotypes,
are called discontinuous characteristics.
Not all characteristics exhibit discontinuous phenotypes. Human height is an example of such a characteristic;
people do not come in just a few distinct heights but, rather,
display a continuum of heights. Indeed, there are so many
possible phenotypes of human height that we must use a
measurement to describe a person’s height. Characteristics
that exhibit a continuous distribution of phenotypes are
termed continuous characteristics. Because such characteristics have many possible phenotypes and must be described
in quantitative terms, continuous characteristics are also
called quantitative characteristics.
Continuous characteristics frequently arise because
genes at many loci interact to produce the phenotypes. When
a single locus with two alleles encodes a characteristic, there
are three genotypes possible: AA, Aa, and aa. With two loci,
each with two alleles, there are 32 = 9 genotypes possible.
The number of genotypes encoding a characteristic is 3n,
where n equals the number of loci with two alleles that influence the characteristic. For example, when a characteristic is
determined by eight loci, each with two alleles, there are
38 = 6561 different genotypes possible for this characteristic. If each genotype produces a different phenotype, many
phenotypes will be possible. The slight differences between
the phenotypes will be indistinguishable, and the characteristic will appear continuous. Characteristics encoded by
genes at many loci are called polygenic characteristics.
The converse of polygeny is pleiotropy, in which one gene
affects multiple characteristics. Many genes exhibit pleiotropy.
Phenylketonuria, mentioned earlier, results from a recessive
allele; persons homozygous for this allele, if untreated, exhibit
mental retardation, blue eyes, and light skin color.